(i) Field of the Invention
The present invention relates to a resin composition for an electrophotographic toner for developing electrostatic charge images in electrophotography, electrostatic recording and electrostatic printing. More specifically, it relates to a resin composition for an electrophotographic toner which can be applied to high-speed machines and which has good balance of fixing/offset properties and excellent durability.
(ii) Description of the Prior Art
In general, an electrophotography for use in a duplicator or a printer is a technique (a heat roll fixing system) which comprises forming an electrostatic latent image on a photosensitive member, developing this latent image by the use of a toner, transferring the toner image onto a sheet to be fixed such as a paper, and then heating/pressing the image with a heat roll. In this technique, the fixing is done under heating and pressing, and so the prompt fixing is possible and a thermal efficiency is extremely high, with the result that a fixing efficiency is very high. However, if this fixing system is utilized with a conventional toner, the toner adheres to and transfers to the surface of the heat roll in a step in which the surface of the heat roll comes in contact with the toner in a melting state. In consequence, the adhered toner further transfers to the next sheet to be fixed, so that the sheet is unpreferably contaminated therewith (an offset phenomenon).
On the other hand, the duplicators have a tendency toward speed-up, and thus a contact time of the fixing roll with the toner is naturally shortened. Therefore, the toner which can be melted by heating for a short period of time is desired. Furthermore, the toner is required which can melt at the lowest possible temperature from the viewpoint of energy saving or safety and which is excellent in fluidity, when melted. In order to obtain the toner having the excellent fluidity, a means for decreasing the molecular weight of a binding resin can be contrived, but such a means is not preferable, because the decrease in the molecular weight makes the cohesion force of the binding resin poor, so that the offset phenomenon is unpreferably liable to occur. If it is attempted to reduce the particle diameter of the toner for the purpose of achieving a high resolving power, a fine powder which is not useful as the toner increases at the time of fine grinding in a toner preparation step, which deteriorates the yield of toner preparation.
Japanese Patent Application Laid-open No. 101031/1974 discloses a technique for obtaining a toner composition which can widen a fixing temperature range by using a crosslinked polymer as the binding resin and which does not give rise to any offset phenomenon even at a relatively high fixing temperature. Furthermore, in Japanese Patent Application Laid-open No. 6895/1980, there is disclosed a technique for obtaining a toner composition with good offset resistance which comprises using, as a binding resin, a polymer meeting the requirements of weight-average molecular weight (hereinafter referred to as "Mw")/number-average molecular weight (hereinafter referred to as "Mn")=3.5-40 and Mn=2,000-30,000. However, the toners obtained by these techniques can scarcely be used in high-speed duplicators and small duplicators in which the amount of heat from the heat fixing roll is not sufficiently transmitted.
That is, if the polymer having the large Mw or the crossliked polymer is used so as to prevent the offset phenomenon, the viscosity of the binding resin increases to deteriorate fixing properties. Conversely, if the polymer having the small Mw or the non-crossliked polymer is used so as to lower the viscosity of the binding resin, an image thermally fixed on a paper is broken by folding or rubbing the paper, so that the toner adhering onto the paper falls down and a defect appears in the image, or image quality is impaired by the offset phenomenon.
As another solvable means, Japanese Patent Application Laid-open No. 75033/1973 discloses an excellent toner using a block copolymer which comprises a hard resin block a polymer portion having a high glass transition temperature (hereinafter referred to as "Tg")! and a soft resin block (a polymer portion having low Tg). However, when this toner using such a block copolymer is used in the duplicator having the heat roll fixing system, the offset phenomenon takes place and it is noticeably difficult to grind the toner in a kneading/grinding method which is a usual preparation method presently employed, so that productivity extremely deteriorates unpreferably.
Additionally, in Japanese Patent Application Laid-open Nos. 88071/1980, 88073/1980 and 90958/1980, some techniques for obtaining a magnetic toner having the excellent offset resistance and fixing properties are disclosed which comprise melting and kneading (1) a styrene polymer, (2) a polystyrene-polybutadiene-polystyrene ABA type block copolymer, (3) a release agent (e.g., polybutene, polybutadiene, a chlorinated paraffin, polyethylene, polypropylene or the like), (4) a magnetic powder and carbon black, and (5) a charging regulator, grinding the kneaded material, and then classifying the resultant particles. In these techniques, however, it is difficult to uniformly disperse the ABA type block copolymer, the release agent, the charging regulator and the like in the styrene polymer in the short-time melting/kneading step, and it has been confirmed that this poor dispersion gives rise to filming and the like, whereby the photosensitive member is easily contaminated. Moreover, if the polystyrene-polybutadienepolystyrene ABA type block copolymer is used in large quantities, a usual grinding by a jet mill or the like cannot be achieved after the kneading, and thus freezing or cold grinding is required.
In order to improve the fixing properties, a technique for obtaining a toner having the heighten offset resistance is disclosed in Japanese Patent Application Laid-open No. 67302/1990 in which a branched polymer is formed in accordance with a suspension polymerization method by the use of a polyfunctional initiator on the basis of the notion that a star-shaped branched polystyrene is more excellent in melting fluidity than a straight-chain polystyrene having the same molecular weight, and the thus formed branched polymer is then used as a high-molecular weight component. Furthermore, Japanese Patent Application Laid-open No. 48657/1990 discloses a technique which comprises forming a high-molecular weight polymer in accordance with a suspension polymerization method by the use of a polyfunctional initiator, forming a low-molecular weight polymer in the presence of the above high-molecular weight polymer, and then using a mixture of these low-molecular weight and high-molecular weight polymers to prepare a toner having the heightened offset resistance.
In Japanese Patent Application Laid-open No. 48675/1990, a technique is disclosed in which a low-molecular weight polymer obtained by a solution polymerization method is dissolved in a polymerizable monomer for a high-molecular weight polymer, and polymerization is then carried out using a polyfunctional (tri- or more-functional) initiator to form a resin for a toner.
As described above, the various techniques have been disclosed, but in the case of the suspension polymerization method, the high-molecular weight resin can be relatively easily obtained by using a crosslinking agent such as divinylbenzene, diethylene glycol dimethacrylate or trimethylol propane dimethacrylate in the polymer preparation step, but in this case, it is necessary to simultaneously use a dispersant and a dispersing agent. Since the dispersant is hygroscopic and have a bad influence on electrical properties, particularly charging stability, they are required to be removed as much as possible after the preparation of the polymer. However, in order to remove the dispersant and the dispersing agent from the polymer by washing or the like, a large amount of washing water is necessary, and after the washing, drainage is also troublesome. These techniques which intend to improve the strength of the toner and the offset resistance are very effective in a conventional duplicator having the problem of the offset phenomenon, but in a low-thermal fixing duplicator, the sufficient low-temperature fixing properties and the satisfactory offset resistance cannot be obtained. This can be presumed to be due to the fact that the toner resin becomes highly viscous because of using the high-molecular weight polymer having large Mw, and the melting fluidity of the toner deteriorates, which is improper to the low-heat quantity duplicator.
If the polymer having small Mw is used, the melting fluidity can be improved, but the offset resistance or the toner strength is poor, so that the toner tends to break or crack during a long-term use to impair image quality.
The present inventors have attempted to improve the strength of a resin by using 0.1 to 4.0 parts by weight of divinylbenzene and an ethylene series polymer having Z average molecular weight (hereinafter referred to as "Mz")/Mn=6 or more and Mw=50,000 or less, as disclosed in Japanese Patent Application Laid-open No. 501873/1991, but in spite of the small amount of divinylbenzene to be used, Mw/Mb becomes large, that is, side chains do not extend sufficiently owing to many crosslinking points, and an entanglement density of molecular chains is also low. Thus, the above-mentioned resin is not so different from a conventional ethylene series polymer.
In addition, the present inventors have developed a resin for the electrophotography toner which is excellent in electrical properties, particularly charging stability by a solution polymerization method, as in U.S. Pat. No. 4,963,456. According to the solution polymerization method, low-volatile components such as an unreacted polymerizable monomer and decomposed materials of an initiator can be all distilled off, when a solvent is removed after the polymerization, and therefore the electrically stable and uniform resin containing very small amounts of impurities can be obtained. This kind of resin is optimum as a resin composition for the electrophotographic toner. However, in the solution polymerization method, a Weissenberg effect (which is the phenomenon that a resin is wound round a stirring rod) tends to appear, and there has been the problem that it is difficult to prepare a high-molecular weight-resin. The present inventors have developed a method for obtaining a resin having higher molecular weight by using a mass polymerization process together with the solution polymerization method, as in U.S. Pat. No. 5,084,368. However, even in the thus prepared high-molecular weight resin, the offset resistance cannot be attained up to a complete level.